1. Field of the Invention
The present invention relates to a modulator for an anti-locking device which prevents the wheels of a vehicle from being locked when the vehicle is braked.
2. Background of the Invention
An anti-locking device is well known which prevents the wheels of a vehicle from being locked when the vehicle is braked. The device includes a modulator which controls the pressure of the braking through the utilization of pressure from a pressure source (such as a pump) which is not the master cylinder, in accordance with a command from an electronic information processor which gives the command on the basis of information from a wheel revolution speed detection means. The technology is well known for dealing with the case that the working fluid of the pressure source differs from that of the brake system, such as where the working fluid of the brake system is vegetable oil and that of the pressure source is mineral oil as the pressure fluid of an engine-driven pump for power steering is utilized. It is also well known how to make the pressure source failsafe. In the art, high pressure from the pressure source is stored except in anti-locking operation and the pressure (hereinafter referred to as control pressure) of a control chamber is reduced depending on the locked state of the wheel in the anti-locking operation to indirectly reduce braking pressure by a pressure reduction means depending on the control pressure. This technique is used instead of directly draining the working fluid from the brake system to reduce the pressure of the working fluid therein and thereafter directly introducing the working fluid from the pressure source into the brake system to again increase the pressure of the working field in the brake system.
FIG. 1 briefly shows an example of the abovementioned conventional art. After pressure produced by a pump 2 driven by an electric motor 1 is accumulated in an accumulator 3, the pressure is alternately applied to a control chamber 5 through an input solenoid valve 4 and discharged from the control chamber 5 to a reservoir 7 through an output solenoid valve 6. When anti-locking operation is not performed (when the input solenoid valve 4 and the output solenoid valve 6 are not supplied with electricity, as shown in FIG. 2), the input solenoid valve 4 is open and the output solenoid valve 6 is closed so that pressure equal to that in the accumulator 3 is introduced as control pressure. A pressure reduction means reduces the pressure of a brake 15 depending on the control pressure. The pressure reduction means includes a pressure reduction piston 8 which has one end located on the control chamber 5 to receive the control pressure and has the other end located on a brake system to receive braking pressure. A valve piston 9 has a valve portion 10 and receives on one end the pressure equal to that in the accumulator 3 and receives on the other end the braking pressure. A body 16 has an inlet port 13 for pressure from a master cylinder 12, an outlet port 14 for the braking pressure to the brake 15 and a valve seat 11 with which the valve portion 10 is brought into contact to constitute a shutoff valve to disconnect the inlet port 13 from the outlet port 14. The body 16 houses the pressure reduction piston 8 and the valve piston 9. The pressure in the accumulator 3 is set to be higher than that in the brake 15, and the pressure reception area A of the pressure reduction piston 8 is larger than the corresponding area B of the valve piston 9. For that reason, when the anti-locking operation is not performed, as shown in FIG. 1, the pressure reduction piston 8 is urged leftward (as illustrated in the drawing), the valve portion 10 is separated from the valve seat 11 to open the shutoff valve, and the pressure produced in the master cylinder 12 is directly applied to the brake 15.
When the braking pressure applied to the brake 15 becomes excessively high and the anti-locking operation is required, the input solenoid valve 4 and the output solenoid valve 6 are suplied with electricity so that the input solenoid valve 4 is closed and the output solenoid valve 6 is opened. As a result, the pressure fluid in the control chamber 5 is drained from the control chamber 5 to reduce the control pressure, the pressure reduction piston 8 is moved toward the control chamber 5 (rightward in the drawing), and the valve portion 10 of the valve piston 9 is brought into contact with the valve seat 11 to close the shutoff valve to disconnect the inlet port 13 and the outlet port 14 from each other. When the control pressure is reduced further, only the pressure reduction piston 8 is moved further toward the control chamber 5 to increase the volume of a closed circuit including the brake 15, to reduce the braking pressure.
When the supply of the electricity is stopped to both the input solenoid valve 4 and the output solenoid valve 6, the pressure in the accumulator 3 is again introduced into the control chamber 5 and the pressure-reduction piston 8 compresses the brake system in accordance with the rise in the control pressure so as to again increase the braking pressure.
The braking pressure necessary for smooth anti-locking operation can be maintained by supplying electricity to only the input solenoid valve 4 to close it to keep the control pressure constant.
Therefore, the modes of pressure reduction, pressure keeping and pressure reincreasing, all which are necessary for the anti-locking operation, can be performed by selectively supplying power to the solenoid valves 4 and 6. Pressure reduction is achieved by supplying electricity to both the solenoid valves 4 and 6; pressure keeping, by supplying electricity to only the input solenoid valve 4; and pressure reincreasing, by not supplying electricity to either of the valves 4 and 6.
If the pressure in the accumulator 3 should fall due to a problem such as a failure, fluid leakage or the like of the pump 2, the urging force acting to the pressure reception area B of the valve piston 9 in such a direction as to close the shutoff valve decreases so that even if the brake 15 is put in action in that state so as to increase the braking pressure over the pressure in the accumulator 3 to move the pressure reduction piston 8 toward the control chamber, the braking pressure acting to the pressure reception area B of the valve piston 9 keeps the shutoff valve in its open position to secure the operation of the brake 15.
However, the above-described conventional art has difficulties as described below. The first problem is that although the closure of the shutoff valve of the pressure reduction means is avoided at the time of the fall in the pressure in the accumulator 3 to secure the connection of the master cylinder 12 and the brake 15, an excessive stroke of the master cylinder is consumed in comparison to the normal operation because of the movement of the pressure reduction piston 8 toward the control chamber 5 so that the controllable braking pressure drops. This excessive stroke is accentuated since the anti-locking operation is usually performed ona plurality of wheels through a plurality of control channels. Therefore, the same number of each of the input solenoid valve 4, the output solenoid valve 6, the control chamber 5 and the pressure reduction means as the control channels are needed. The second problem is hereafter described. The continuously rotating type of a pressure source for anti-locking operation uses an engine-driven pump or the like. Therefore a mechanical means such as an unloader valve, is needed for preventing the pressure of the pressure source from becoming higher than a prescribed level. In the emergency rotation type of a pressure source, a pump is driven by an electric motor. It is thus necessary to use an electric pressure switch to keep the pressure of the pressure source constant. In the abovedescribed conventional art wherein the pressure of the control chamber needs to be always kept high except in the anti-locking operation it is necessary to provide a mechanical relief valve to prevent a pressure rise due to a temperature rise or to make an electric pressure control means failsafe so as to preclude an abnormal rise in the pressure of the pressure source. For that reason, the design for practicing the art is complicated, the reliability of the resultant structure is low, and its cost is high. This is the second problem.